Growth and Functional Analyses of Photosynthetic Strategies of Prochlorococcus Strains Under Varying Oxygen

Julie Nadeau1, Mireille Savoie1, Douglas Campbell1


1 Department of Biochemistry, Mount Allison University

Introduction

[1] Prochlorococcus:

  • Photosynthetic unicellular cyanobacterium
  • Contributes most primary production in the open oceans.
  • Different strains occupy a wide range of habitats, including Oxygen Minimum Zones.
  • With climate change, ocean warming will benefit Prochlorococcus, but also cause decreased oxygen solubility.
    The biological interactions of *Prochlorococcus* with oxygen and light... Adapted from (Cai et al., 2023).

    Figure 1: The biological interactions of Prochlorococcus with oxygen and light… Adapted from (Cai et al., 2023).

Objectives

  1. Determine whether Prochlorococcus strains are constitutively able to accomodate changes in oxygen, or whether they acclimate over a period of time to different levels of oxygen.
  2. Provide insights into the potential ecological niches of Prochlorococcus strains.

Methods

Bioptical Analysis of Growth Rates

Using a Multi-Cultivator, two strains of Prochlorococcus (MED4, MIT9313) were monitored for OD680 (Chlorophyll and scattering) and OD720 (cell scattering). under 22°C, 12h photoperiod of blue light (450 ± 45 nm), and combinations of:

  • Dissolved O2
    • 250, 25, 2 µM

  • Light level

    • 30, 90, 180 µmol photons m-2 s-1

Bioptical Functional Measurements

  • Exposed samples under 250, 25, 2 µM O2, and a series of increasing light levels to track ‘light response’ curves of Photosystem II electron transport, using Solisense FRRf Instrument.
  • Photosystem I and Photosystem II electron transport in parallel, using Dual-PAM-100 Instrument.

Results & Discussion

Light Response Curves of PSII electron transport (e- PSII-1 s-1) vs measurement light (µmol photons m-2 s-1) for *Prochlorococcus* strains MED4 and MIT9313, after growth under combinations of light level (rows, 30, 90, 180 µmol photons m-2 s-1) and oxygen concentration (columns, 2, 25, 250 µM). The curves were measured under 2 (red), 25 (green), or 250 (blue) µM O~2~. Lines show Platt curve fits (citation).

Figure 2: Light Response Curves of PSII electron transport (e- PSII-1 s-1) vs measurement light (µmol photons m-2 s-1) for Prochlorococcus strains MED4 and MIT9313, after growth under combinations of light level (rows, 30, 90, 180 µmol photons m-2 s-1) and oxygen concentration (columns, 2, 25, 250 µM). The curves were measured under 2 (red), 25 (green), or 250 (blue) µM O2. Lines show Platt curve fits (citation).

  • MED4 and MIT9313 show significant short term responses of electron transport to decreasing oxygen.
  • Growth under 2 µM O2 diminishes the short term effects of changing measurement oxygen, indicating growth acclimation to oxygen status.
Maximum PSII Electron Transport Rate (Pmax) for *Prochlorococcus* MED4 and MIT9313, derived from light response curve fits, vs. measurement oxygen concentrations (error bars = SE). Data is grouped by strain and growth oxygen concentration (columns), and by growth light levels (rows).

Figure 3: Maximum PSII Electron Transport Rate (Pmax) for Prochlorococcus MED4 and MIT9313, derived from light response curve fits, vs. measurement oxygen concentrations (error bars = SE). Data is grouped by strain and growth oxygen concentration (columns), and by growth light levels (rows).

  • Strain MED4 shows increasing Pmax values across increasing measurement oxygen concentrations, indicating short term responses to varying oxygen levels. Pmax also incresses with increasing light levels and with growth at 25 µM O2. Consistent with (citation).
  • Strain MIT9313 shows interactive effects of measurement oxygen, growth oxygen concentration and growth light on Pmax values. particularly between the lowest (2 µM) and highest (250 µM) oxygen concentrations, indicating acclimating adaptation to varying oxygen levels.
  • These findings underscore the influence of longterm growth and short term oxygen concentrations on the PSII electron transport capacity of Prochlorococcus strains, with significant changes in Pmax values indicating cellular capability to acclimate to different oxygen concentrations over time.
PSI electron transport (e- PSI^-1^ s^-1^) vs PSII electron transport (e- PSII^-1 s^-1) for *Prochlorococcus* strains MED4 and MIT9313 after growth under different combinations of light level (30, 90 µmol photons m^-2 s^-1) and oxygen concentration (2, 25, 250 µM). Mesurement under 2.5 (red), 25 (blue), or 250 (green) µM O~2~.

Figure 4: PSI electron transport (e- PSI-1 s-1) vs PSII electron transport (e- PSII^-1 s^-1) for Prochlorococcus strains MED4 and MIT9313 after growth under different combinations of light level (30, 90 µmol photons m^-2 s^-1) and oxygen concentration (2, 25, 250 µM). Mesurement under 2.5 (red), 25 (blue), or 250 (green) µM O2.

  • Directly comparing PSI to PSII electron transport shows that in MED4 growth under 25 µM O2 decreases PSI electron transport.
  • In contrast, in MIT9313, PSI electron transport remains more consistent across growth O2 concentration.

Next Steps

  • Cell pellets were obtained by centrifuging samples, frozen in liquid nitrogen, and preserved in an -80°C freezer, facilitating future transcriptomic analyses.
  • Whole Cell Absorbance Spectra were measured using an Integrating Cavity Spectrophotometer, at 250 or 2 µM O2, to detect changes in the reduction status of electron carriers.
  • Chlorophyll a concentrations were determined for each sample, aiding in the characterization of photosynthetic pigment content and its relation to oxygen levels.

Conclusion

Prochlorococcus shows both long and short term responses to oxygen.

References

1.
Patil PP, Vass I, Kodru S, Szabó M. A multi-parametric screening platform for photosynthetic trait characterization of microalgae and cyanobacteria under inorganic carbon limitation. PLOS ONE. 2020;15: e0236188. doi:10.1371/journal.pone.0236188